The present invention relates to optical modulators and, in particular, it concerns a linearized optical digital-to-analog modulator.
There is a tangible need for high-performance and large bandwidth digital to analog signal conversion. Furthermore, as the RF and digital domains converge, entirely new solutions will be needed to enable multi-GHz mixed-signal systems. Probably the most prominent area to benefit is the wireless communication industry. The ever increasing thirst for bandwidth will require data converters to deliver greatly increased performance. For example, analog signals are transmitted in cable television (CATV) via optical fibers and the demand for increasing bandwidth is driving technology to speed-up the processing of signals as well as the transmission. High performance digital to analog conversion is also required to address the growing demands of wireless carriers for supporting the heavy traffic expected in the base station. Additional specific areas to benefit include: the defense and government industries that concentrate on deploying multi-function, dynamically reconfigurable systems (RADAR, electronic warfare, and surveillance applications); medical imaging; and hyper/super-computer communications.
One of the most widely deployed devices for analog optics modulation is the Mach-Zehnder Interferometer modulator (MZI). For binary digital signals, it is today the preferred device for long-haul fiber-optic communication, leading to chirp-free pulses which can reach hundreds of kilometers in optical fibers without the need for regeneration. For analog applications, however, a serious problem is encountered due to the inherent non-linear response of the modulator. Specifically, since the modulating voltage via the electro-optic effect controls the optical phase delay in a basically linear fashion and the attenuation varies as the cosine of the phase difference between the two branches of the device, a linear variation in phase difference and thus in applied voltage results in a cosine-shaped output variation, as seen in the pattern of points in FIG. 2A. The common solutions for this problem are either the biasing of the device to a quasi linear regime coupled with reducing the modulation range to reduce distortion, or use of an analog pre-distortion circuit to feed the modulator. Since in practically all present systems signals are processed digitally, a multi-bit Digital-to-Analog Converter (DAC) device is needed with fast processing capabilities.
A DAC based on a multi-electrode MZI modulator concept was proposed many years ago by Papuchon et al. and is described in U.S. Pat. No. 4,288,785. In that device, the electrodes' sectioning length followed a conventional power-of-two digital sequence, which did not solve the non-linearity problem, and thus suffered from severe limitation in the dynamic range, and subsequently the attainable resolution. More recently, much more complex devices have been presented to cope with these problems: Yacoubian et al. (“Digital-to-analog conversion using electrooptic modulators,” IEEE Photonics Technology Letters, vol. 15, pp. 117-119, Jan. 2003), proposed the employment of one MZI modulator for each and every bit. A recently reported design by Leven et al. (“A 12.5 gsamples/s optical digital-to-analog converter with 3.8 effective bits,” Lasers and Electro-Optics Society, 2004. LEOS 2004. The 17th Annual Meeting of the IEEE, vol. 1, pp. 270-271, Nov. 2004), also the subject of U.S. Pat. No. 7,061,414 entitled “Optical Digital-To-Analog Converter” to Y K Chen et al., employs a single modulator for every 2 bits and is highly nonlinear; it yields only 3.8 effective bits for a 6 bit design. There is therefore a need for a digital to analog converter which would offer improved linearity of response without sacrificing efficiency or dynamic range.